Radiofrequency amplifier



April 4, 1933- F. K. VREELAND 1,903,394

RADIOFREQUENCY AMPLIFIER Original Filed Aug. 6, 1923 3 Sheets-Sheet l R Q. n c: k

E my E INVENTOR I M @AW/ April 4, 9 F. K. VREELAND 1,903,394

RADI OFREQUENGY AMPLIFIER Original Filed Aug. 6, 1923 3 Sheets-Sheet 2 April 4, 1933. F. K. VREELAND RADIOFREQUENCY AMPLIFIER Original Filed Aug. 6, 1923 3 Sheets-Sheet NO/ibW/J/ 70 1449 Patented Apr. 4, 1933 UNITED STATES PATENT OFFICE FREDERICK K. 'VREELAND, OF MONTCLAIR, NEW JERSEY, ASSIGNOR TO VREELAND CORPORATION, OF HOBOKEN, NEW JERSEY, A CORPORATION OF NEW JERSEY BADIOFREQUENCY AMPLIFIER Original application filed August 6, 1923, Serial No. 655,794, patented September 4, 1928, No. 1,682,874. Divided and this application filed August 9, 1928. Serial No. 298,586.

This application is a division of application Serial No. 655,794 filed August 6, 1923, on which has issued Patent No. 1,682,874, Sept. l, 1928.

The invention herein described relates to amplifiers for high or radio frequency cur rents and is particularly concerned with de vices for coupling amplifier tubes in cascade.

It has for one of its objects the elimination or minimizing of the effects of detrimental capacity and stray fields, and the production of an amplifier of greatly improved efficiency.

In accordance with one feature of the invention the coupling transformer is rendered practically astatic, so that the effect of stray fields is eliminated or minimized.

In accordance with another feature of the invention the arrangement of polarities in the windings is such that the tendency to regeneration or oscillation is minimized. Other features are set forth at length herein.

In the drawings Fig. 1 is a schematic diagram showing a simple form of apparatus embodying the invention. Figs. 1a, 1b and lo show alternative arrangements of certain features of Figs. 1, Zand 6.

Figure 2 is a modified form in which the astatic feature is included.

Fig. 3 is a more detailed diagram showing the physical construction of one type of coupling transformer.

Figs. 4 and 4a show the structural features of another type of transformer embodying the invention.

Fig. 5 shows the amplification characteristics secured by application of the invention.

Fig. 6 is a schematic diagram of a multiple band amplifier.

One of the most serious limitations in the design of radio frequency transformers, as usually practiced, is the fact that the secondary electromotive force is largely dissipated in stray capacities, so thatonly a fraction, and sometimes a smal fraction, of it is useful in producing amplification. Usually it is considered impracticable to construct such a transformer having a step-up ratio, because the electromotive force gained by the step-up 59 ratio is fritteredaway in these stray capacities, including the capacity of the transformer itself. Such strayor effective natural capacities always exist between the coils, condensers and other parts of the apparatus and at radio frequencies their effects are important.

By means of the present invention I not only avoid the detrimental effect of capacity in the windings, but I utilize the capacity to produce an improved result. By means of this invention I am enabled to construct a transformer of high efficiency with a step-up ratio.

Another difficulty in radio frequency amplifiers of the usual type is inter-action of the various stages, when amplifiers are used in cascade, which reduces the effective amplification and tends to instability or oscillation of the system. An important feature of the present invention is an arrangement of windings whereby the external field of the transformer is reduced to a minimum so that it becomes practically astatic and inductive disturbances are eliminated.

In the drawings Fig. 1 shows schematically a simple embodiment of the invention, as applied to a radio frequency amplifier of three tubes. In the arrangement shown, two of the tubes A A are used for radio frequency amplification and the third D as a detector. When in the following I refer to amplifier tubes generically it will be understood that one of these may be used for detection as shown. T, T are transformers coupling the tubes in cascade, each including a primary coil p and a secondary coil 8. In the first transformer the outer terminal of the pri mary coil p is connected to the output terminal or anode of the first amplifier tube A and the outer terminal of the secondary s is connected. to the input terminal or grid of the second amplifier tube A The second transformer is similarly connected to the sec ond amplifier tube A and the detector tube D. The primary and secondary coils are wound or connected in opposite senses so that their outer terminals (1 and b respectively are at opposite potentials, instead of being at like potentials as is usual in the construction of such transformers.

This feature is 100 explained.

pacity augments or assists the magnetic cou pling. effect of thetwo windings, as will be This inherent capacity of the windings is indicated by the dotted capacity couplings C in the drawings, dotted lines 7 being employed to indicate that the capacities, although physically and necessarily existent as capacities, do" not necessarily require separate structural elements, as usually employed, but may be inherentin the transformer structure; If desired an external condenser maybe connected across the terminalsa b to augment the inherent capacity of the transformer windings as shown at 8 in Fig. 10. This capacity, whether inherent in the windings or external, assists the magnetic coupling of the transformer. It may be sochosen that its effective reactance at the preferred operating frequency is approximately equal to the combined reactances of the coils 798, so that the transformer becomes in effect an oscillating circuit 125C, tuned approximately to suchfrequency, due allowance beingmade for the capacities of the associated tubes when these are material with respect to the capacity of the transformer. The result of this arrangement is that the full potential difference between the terminals a Z) is effective in coupling the output of one tube with the input of the next. It will be understood that the capacity referred to, whether inherent in the transformer structure or provided in whole or in part by a separate structural element, is not detrimental, as are the coil'capacities in other arrangements, but is useful in increasing the effectiveness of the device inasmuch as under the presentinvention the capacity coupling tends to augment or assist the inductive coupling of the windings and its capacity reactance tends to neutralize or may completely neutralize the inductance reactance of the windings.

I The windings ps may be similar, in which case the useful secondary potential will be muchlarger than that of-anequ al ratio transformer not embodying this invention, but preferably the secondary is made with more turns than the primary so that a step-up ratio is secured. 1

The significance of this arrangement will be seen more clearly by reference to Fig. 1a, WhlCl'l 1s a simplified schematic diagram showing the relations of one pair of tubes to the coupling system. The inductance of the coupling system comprises the inductancev .electromotive force is taken off between points 6 and 0.. The ratio between the input and output electromotive forces is determined by the inductivedrops across coils and s, which may be given any desired value within wide practical limits by suitab y proportioning the coils.

To secure the best results it is usually desirable to shunt the external connections of the transformers including batteries, resistances, etc.,-by bridging condensers such as 0 whose capacity reactance in the working range of frequency issmall. These serve to by-pass-the high frequency oscillations across the inner terminals of the transformers, avoiding external loss by completing the closed circuit including the'windings 79s and the mutual capacity of the coils by the shortest possible route. Otheribridging condensers such as c may be used when desired to by-, pass the output current of one tube and the input current of the next to the filament or ground 7 bus. i

Fig. 2 shows an improved arrangement 1 V which includes the features contained 1n Fig. 1 and has in addition an astatic feature whereby the external field of the transformer is practically eliminated. In this arrangement each of the coils of the transformer is made in two sections, wound or connected in opposite directions so that their external fields oppose and neutralize each other. In the primary the'two sections are 20 p and in the secondary the two sections are 8, The section 79 is arranged preferably in close inductive relation with the section 8 and the section in close inductive relae tion with the section 8 so that a relatively close coupling is obtained between primary and secondary through their unbalanced in ternal fields, notwithstanding the opposing relations of the two halves of these windings which cause their external fields to balance and neutralize each other. The sections p 8 are wound or connected in opposite senses, as explained in connection with Fig. 1, and the same is true of the sections p 5 so that the external terminals a Z) will have opposite polarities as in the arrangem at of quency approximately equal to the inductance reactance, so that at this preferred frequency the maximum effective potential difference is obtained between terminals a b.

In this arrangement it will be noted that while the mutual induction of the two windings is relatively large, their external magnetic field is negligible. Since the potentials of the terminals at are opposite, the stray electrostatic fields are also reduced to a minimum so that undesired couplings'are prac' tically eliminated. This arrangement of windings also ofi'sets the reversal of polarity occurring in the amplifier tube, so that all tubes have like polarities, and the tendency to regeneration or oscillation by mutual interaction is avoided.

There are various forms of construction that can be used in carrying out the invention. One of these is shownin Fig. 3, in which cylindrical or solenoidal windings are used. Here the primary coil is made in two halves, p p wound in opposite directions as shown, and the secondary coil is also wound in two opposing halves s .9 as indicated. The primary and secondary coils are shown separated for clearness of illustration of the windings. In operation the primary coil is placed inside the secondary, or vice versa, and the dimensions of the windings and diameters of the tubes are so chosen that the capacity between windings has the requisite value. The drawings show also the direction of winding of the coils which gives the opposite potentials at the external terminals a I).

Figs. 4 and 40 show another type of wind ing in which the coils are in disc form. These coils are wound preferably in slots of suitable thickness between three insulating discs E, clamped together with central spacers F forming a core on which the coils are wound. The slot for the secondary is preferably made wider than that of the primary so that the secondary has a larger number of turns. The coils are constructed by winding first the desired number of turns in one direction for the inner half of the coil, as 7 then winding silk thread to form v a spacer G and finally winding the second section of the coil p in the opposite direction. The same construction is used in the secondary coils s 8 though in this case preferably the number of turns in each section is greater.

The dimensions of the coils and the distance between them are so chosen that the capacity has the desired value in relation to the inductance of the windings.

Either of the constructions shown in Figs. 3, 4 and 4a may be used for the transformers TT in Figs. 2 and 6, or any other suitable construction preferably embodying the feature herein described, may be used.

The principle of operation of this invention is capable of a wide variety ofapplications.

By winding the transformer coils with low resistance the apparatus may be made highly selective, so that the transformer circuit with its inherent and associated capacities is resonant to a definite frequency and the amplification is high for this frequency but low for any other frequency. The characteristic curve of such a transformer is shown at a in Fig. 5, in which the ordinates represent the ratio of amplification and the abscissas represent the frequency.

On the other hand, where an amplifier is desired which works effectively over a wide range of frequencies I prefer to wind the transformer coils, or particularly the secondary coil, of relatively high resistance; for example by winding them of very fine copper or larger German silver wire. Or the resistance may be inserted externally; for example, it may be placed effectively in' series with the bridging condenser C between the inner or low potential terminals of the windings, as shown at R, Fig. 1b. This resistance, whether inherent in the winding or inserted, has the effect of lowering the peak of the amplification curve without materially lowering its amplitude at frequencies different from the resonant frequency, so that a broad characteristic is secured without corresponding loss in efiiciency, as shown for example at b in Fig. 5. A certain amount of resistance is also effec tive in preventing the tendency to regenerate or oscillate which may occur when a number of amplifiers are used in cascade.

The degree of coupling between the coils is also an important factor in determining the amplifier characteristics. Vhere a broad characteristic is desired I prefer to make the coupling as close as practicable. Where a sharply selective characteristic is desired a looser coupling is permissible. It will be understood that the use of a loose coupling is equivalent to increasing the effective ratio of turns. By suitably proportioning the coupling and resistance, as well as the inductance and mutual capacity of the windings, the characteristics of the amplifier may be determined at will. The factors set forth in the foregoing for securing a sharply selective characteristic, namely, employing a step-up ratio of turns, and increasing the effective ratio of turns by decreasing the coefiicient of coupling, have the result of increasing the input conductance at resonance as will be readily understood. It will be understood also that any conductance shunted across a resonant circuit, such as the plate conductance of the associated vacuum tube, will increase the damping and decrease the selectivity of such a circuit, unless the shunted conductance is substantially lower than the input conductance of such circuit across which such shunted conductance is connected. A. substantially flat combined or Overall characteristic may be obtained over a desired band of frequencies by the use of amplifier units in cascade having individual characteristics which are difierent. For example, Fig. 5, at c shows a typical-characteristic of a three stage band amplifier. The curves in broken lines 1,2, 3 show the characteristics of the three individual units including tubes and transformers or other coupling means that are connected in cascade, while the full line curve l represents the resultant or overall band characteristic, of the combined system.

The different characteristics of the individual units of a band amplifier may be determined by the design of. the windings, or characteristics like those shown in 1, 2', 3 at c in Fig. 5 may be obtained from transformers of. identical windings by suitably choosing the distance between the'coils and the resulting mutual capacity or by other suitable means. Separating the coils by a dielectric spacer or otherwise raises the peak fro quency of the characteristic curve by diminishing the capacity and the coupling.

Another convenient means of adjusting the characteristic of a transformer at will is by the use of condensers. Three ways of doing this are shown in Fig. 1c. The first is by shunting an external condenser 8 across the outer terminals a,'b, of the primary and secondary coils p 10 s 8 This external capacity is in effect added to the inherent mutual capacity of the coils, and so lowers the frequency characteristic. The second arrangement consists in connecting a condenser 9 in parallelwith the primary coil 19 29 This has a result equivalent to increasing the effective inductance of the primary coil and consequently'it lowers the frequency characteristic A similar result is obtained by connecting a condenser 10 across the second-. ary coil 8 8 but'the efiect of such capacity is proportionately greater than when the same capacity is added at 9 if, as I prefer, the transformer coils 20 p s .9 have a stepup. ratio; r

The condensers or capacities 8, 9, 10 may be used singly or in any desired combina-' tion, and any or all of them may be made variable. An effective arrangement for tuning the coupling transformers is secured by designing the transformers to have a sharply selective characteristic and inserting a variable condenser at 9. v V

A particularly useful modification of the band amplifier is shown in Fig. 6. Here are three transformers with their corresponding tubes arranged in cascade. These transformers are preferably designed to have overlapping characteristics as shown at c in Fig. 5,

giving acombined band characteristic which is flat. It will beobserved from this figure that when the spacing of the individual char;

acteristics is suitably .related to the width or form of the characteristics, the resultant characteristics of over-all amplification is a substantialiy flat band with a sharp cut-off at each side. The desired spacing may be secur-ed by any of the means herein described for determining the frequency characteristic of the amplifier, or by any other suitable means. This characteristic is shifted in the frequency coordinate by adding impedances to the transformer systems, preferably by the use of smallcondensers 9 9 of suitably proportioned capacity so arranged that they may be shunted successively across the transformers. These condensers are cut in and out by switches S S, the switches S S B being arranged to be operated simultaneously, for example by mounting them on a common rock shaft 11 operated by a handle or lever 12'. Similarly theswitches S S S oper te simultaneously. When the switches S are closed by rockingthe shaft, cutting in the condensers 9 the characteristics of all the transformers are changed in frequency so that the combined characteristic is a flat toppedband displaced inthe frequency co ordinate as shown at 6 in part cl Fig. 5; Additional condensers 9" may be switched on by a further movement of the rockshaft 11, closingthe switches S, and in this way the operating range of the amplifier may be broadened as far as desired. Preferably the capacities of the condensers 9 9 are so chosen that the bands 5, 6, 7 have a slight overlap, as shown. v I

This arrangement has numerous advantag'esv An amplifier is secured which gives practically uniform ampiification over a wide rangc of frequencies, and the sub-division of this range into a plurality of narrower bands is useful in eliminating interference from signals of undesired frequencies, since the ampiifier is highly effective for frequencies within its hand but excludes almost complete iy frequencies outside of this band. By this means extremely feeble signals may be received without interference from the most powerful nearby stations.

While I prefer to shunt the band-shifting condensers 9, 9 across the primary coils 20 10 as shown, they may be placed,if desired, in any of the positions 8,9, 10, shown in Fig. 10. e

The flatness of the band of effective amplification and the sharpness of the cut- 0E for frequencies outside this hand, which characteriz'e the preferred form of my invention, are secured by sorelating the spacing of the characteristics of the severalainplifier stages to the width and form of the characteristics as to secure the particular overlap that is re- Considering the relation of these curves at difierent frequencies, it is noted that for a frequency less than the peak frequency of the characteristic 1, characteristic 3 shows substantially no amplification and the overall characteristic is small. As the frequency is increased, all the component characteristics are increasing simultaneously, and the am-.

plification rises very sharply up to a frequency corresponding to the peak of curve 1. At this frequency curve 1 begins to fall while the others rise more slowly, and, by virtue of the particular spacing employed, the rise of one curve substantially offsets the fall of another, so that the amplification is substantially uniform, up to a point corresponding approximately to the peak of curve 3, beyond which all of the curves are descending. The resultant characteristic descends with a very sharp cut-off.

It will be noted that while all of the component characteristics are selective, their cutoff is gradual in the manner that is usual in resonance curves. In the case of the overall band characteristic 4:, however, the cutoff is sharp-much sharper than the slope of the component characteristicsso that the width of the curve at its base is not materially greater than that of the component characteristics, notwithstanding the breadth and flatness of the band at its top.

lhis is of great importance since it permits a high degree of selectivity due to the sharp cut-off, notwithstanding the width of the band of effective amplification.

The flatness of the band depends upon the spacing. If the spacing is closer than the optimum, the over-all characteristic will be peaked. If the spacing is broader than the optimum, the over-all characteristic will have a central valley. Usually the optimum spacing is secured for a 3-stage amplifier when the high and low frequency character- H istics (3 and 1, Fig. 5, c intersect and overlapat a point corresponding to about half the maximum amplification. For other amplifiers the optimum spacing is readily determined by computation or graphic methods.

What I claim is:

1. In an amplifier for hi h frequency currents, a plurality of ampli er tubes, a polarity reversing inductive coupling between these tubes, and a capacity shunting the polarity reversing inductive coupling.

2. In an amplifier for high frequency currents, a plurality of amplifier tubes, a polarity reversing inductive coupling between N these tubes and a capacity whose reactance is approximately equal to the inductance reactance of the coupling means shunting the polarity reversing inductive coupling.

3'. In an amplifier for high frequency currents, a plurality of amplifier tubes and a polarity reversing inductive coupling between these tubes comprising coupled inductances having an inherent mutual capacity tending to assist the magnetic coupling of the inductances.

4. In an amplifier for hi rh frequency currents, a plurality of amplifi er tubes, a polarity reversing inductive coupling between these tubes comprising closely coupled inductances having an inherent capacity tending to neutralize the reactance of the inductances. I

5. In an amplifier for hi h frequency currents, a plurality of amplifier tubes, a polarity reversing inductive coupling between these tubes comprising coupled inductance coils having a step-up ratio and an inherent capacity tending to neutralize the reactance of the inductances.

6. In an amplifier for high frequency currents, a plurality of amplifier tubes, coupling transformers connecting the amplifier tubes in cascade, having reversed windings where'- by the reversal of polarity that is inherent in the tubes is again reversed in the coupling means, so that all the successive tubes have similar polarities.

7. In an amplifier for high frequency'currents, a plurality of amplifier tubes, coupling transformers connecting the amplifier tubes in cascade, having reversed windings where by the reversal of polarity that is inherent in the tubes is again reversed in the coupling means, so that all the successive tubes have similar polarities, and capacities shunting the windings of the transformers.

8. In an amplifier for high frequency cur rents, a plurality of amplifier tubes, coupling transformers connecting the amplifier tubes in cascade, having reversed windings whereby the reversal of polarity that is inherent in the tubes is again reversed in the coupling 9. In an amplifier for high frequency cur rents, a plurality of amplifier tubes, coupling transformers connecting the amplifier tubes in cascade, having reversed windings where'- by the reversal of polarity that is inherent in the tubes is again reversed in the coupling means, so that all the successive tubes have similar polarities, the windings of the transformers being so proportioned and arranged as to have a mutual capacit tending to as sist the magnetic coupling. 0 the windings.-

10. In an amplifier for high frequency currents, a plurality of amplifier tubes, coupling transformers connecting the am lifier tubes in cascade, having reversed win ings whereby the reversal of polarity thatis inherent in the tubes is again reversed in the coupling fob fin

means, so that all the successive tubes have similar polarities, the windings of the transformers being so proportioned and arranged {in two sections inductively coupled I and .WOUJld so that their outer terminals are of opposite potentials, a capacity shunting the inductance having a reactance approximately' equal to the inductance reactance and a neutral or .bus connection to the sections between the input and output terminals.

12. In an amplifier for high frequency currents, a plurality of amplifier tubes and between these. tubes a coupling coil connecting these tubes in cascade and woundin sections Whose external fields are mutually reversed, these sections being so arranged that their external fields oppose and neutralize each other.

13. In an amplifier for high frequency currents, a plurality of amplifier tubes and between these tubes a coupling transformer connecting these tubes incascade and comprising primary and secondary coils each wound in two sections whose external fields are mutually reversed, each section of the primary being coupled to the corresponding section of the secondary, these couplings being so arranged as to assist each other while their mutually reversed external fields oppose and neutralizeeach other.

14:. In an amplifier, for high frequency currents, a plurality of amplifier tubes and between these tubes a coupling transformer connecting these tubes in cascade and comprising primary and secondary coils each wound in two sections whose external fields are mutually reversed, each section of the primary being coupled to the corresponding section of the secondary, these couplings being so arranged as to assist eachv other while their mutually reversed external fields op pose and neutralize each other, the primary and secondary coils being wound to produce opposite potentials at the input and output terminals.

15. In an amplifier for high frequency currents, a plurality of amplifier tubes and between these tubes a coupling transformer connecting thesetubes in cascade and comprising primary and secondary coils each wound in two sections whose external fields are mutually reversed, each section of the primary being coupled to the corresponding sectionof the secondary, these couplings being so arranged as to assist each other while their mutually reversed external fields opposeand neutralize each other, and'a capacity shunting thewindings of thetransformer. r Y

i 16. In. an amplifier for high frequency cure rents, a plurality of amplifier tubes and between these tubes a coupling transformer connecting these tubes in cascade and comprising primary and secondary coils each woundin two sections whose external fields are mutually reversed, each section of the primary being coupled to the corresponding section of the secondary, these couplings being so ar-' ranged as to assist each other while their mutually reversed external fields oppose and neutralize each other,.the primary'and sec ondary coils being wound to produce opposite potentials at the input and output terminals, and a capacity whose reactance is approximately equal to the effective inductance reactance of the transformer windings at the preferred working frequency. 7

17. In an amplifier forhigh frequency currents, a plurality of amplifier tubes, polarity reversinginductive coupling means connecting these tubes in cascade, each coupling means having a capacity shunting the windings which substantially neutralizes the inductance reactance at a given frequency, said frequencies being different for the several coupling means which haveoverlapping response curves so related to each other that the combined amplifying effect of the system is substantially uniform over a given band of. frequencies.

18. In an amplifier for high frequency currents, a pluralityof amplifier tubes, coils con necting these tubes in cascade, each coil being wound in sections whose, external fields opposeand neutralize each other, and a capacity shunting each coupling coil which substantially neutralizes the effective inductance reactance at a given frequency, a said frequencies being different for the several coils which have overlapping response curves so related to each other that the combined amplifying effect of the system is substantially uniform over a given band of frequencies.

19. The method of amplifying high frequency currents which consists in amplifying the current energy,'inductively revers-- ing the polarity of {the amplified output, shunting the direct and reversed polarity points by a capacity, and combinlng cumu-Y latively the inductance and capacity, output.

20. The method of amplifying-high fre-' quency currents which consists in amplifying thecurrent energy, inductively reversing the polarity of the I. amplified output, shunting the direct and reversed polarity points by a capacity, thereby neutralizing the polarity reversing inductance, andcombining cumulatively the inductance and capacity output t 21. The method of amplifying high frequency currents which consistsinamplifying the current energ ,inductively reversing the polarity of the amplified output, and shunting the reversing inductance by a capacity Whose reactance is approximately equal to the reactance of the reversing inductance.

22. The method of amplifying high frequency currents which consists in amplifying the current energy, producing by the output energy a polarity reversing induction current and a substantially equal and opposite capacity current, producing by combination of these currents an increased reversed potential, and utilizing this potential.

28. In an amplifier for high frequency currents, a plurality of amplifier tubes, a coupling between the output terminal of one tube and the input terminal of the next tube, and an astatic inductance included in said coupling having sections Wound and related in opposite senses to cause their external fields to oppose and neutralize each other, thus avoiding undesired coupling, and to have an unbalanced internal field constituting an effective inductance.

24. In an amplifier for high frequency currents, a plurality of amplifier tubes, a coupling between the output terminal of one tube and the input terminal of the next tube,

and an astatic inductance included in said coupling made in four sections inductively coupled in pairs so that the input and output terminals are at opposite potentials, the coupled pairs being Wound and related in opposite senses to cause their external fields to oppose and neutralize each other, thus avoiding undesired coupling, and their internal fields to constitute an effective inductive coupling.

This specification signed this 26th day of July, A. D. 1928.

FREDERICK K. VREELAND. 

